Field of the Invention
The present invention relates generally to private radio communication systems, which typically cover local indoor residential or business areas. Particularly, the present invention relates to radio communication systems which employ an air-interface compatible to an existing cellular digital Time-Division Multiple Access (TDMA) standard like the Global System for Mobile Communication (GSM) or the Digital Advanced Mobile Telephone Service (D-AMPS). More particularly, the present invention relates to automatic frequency allocation between uncoordinated radio communication systems sharing radio spectrum, particularly between public micro/pico systems or private pico systems sharing frequencies with an overlapping macro cellular network that applies frequency hopping, and methods and communication systems to effectuate the same.
The past decades have seen a considerable rise in the deployment of mobile telephony. After the slow start of analog standards like AMPS, Nordic Mobile Telephone (NMT) and the Total Access Communication System (TACS), mobile telephony has recently become quite popular in the consumer markets with products employing advanced digital standards like GSM and D-AMPS. In addition to other developments in mobile phone features, like smaller size and longer battery life, much progress has been made at the network side as well, particularly, in frequency reuse schemes to avoid co-channel interference between adjacent cells. Increasingly, dense cell reuse plans have been complemented with hierarchical cell structures, where macrocells cover entire districts (high tiers), microcells cover smaller parts like streets (middle tiers), and picocells cover very small areas the size of a few rooms (low tiers). Important for the hierarchical cell structure is that all the base stations deployed (ranging from macro to pico base stations) are part of the same public land mobile network (PLMN).
In order to avoid co-channel interference between different radio links, a structured channel allocation scheme is applied, i.e., different sets of frequencies are assigned to different tiers with frequency reuse being applied within the respective tiers to maximize radio spectrum usage. Public telephone systems typically operate in a licensed part of the radio spectrum, typically in the 1 GHz and 2 GHz range. The radio spectrum is divided in to two or more bands which are assigned to different operators. One band contains a multiple of radio channels, each centered around a different radio carrier frequency. In a restricted local area, a multitude of mobile users can be served by allocating different radio channels to them. In this way, no cross-interference between the users occurs. This is called Frequency Division Multiple Access (FDMA), and is applied in all analog mobile telephone systems. In the more advanced digital systems, each radio channel contains a number of subchannels in the form of time slots, e.g., FDMA/Time Division Multiple Access (FDMA/TDMA) or a spreading code, e.g., FDMA/Code Division Multiple Access (FDMA/CDMA). Each subchannel can serve a different user. However, since the assigned band is limited, only a limited amount of users could be served with this concept. Frequency reuse has been introduced to solve this limitation: the same radio frequencies are reused at different geographical locations such that minimal cross-interference (co-channel interference) occurs. If the propagation loss between a mobile and its serving base is much smaller than the propagation loss between this mobile and an interfering base, the carrier-to-interference ratio (C/I) at the mobile receiver is sufficient for acceptable link quality.
It should, therefore, be understood that radio systems covering the same geographical area must make use of different frequencies in order to avoid the aforedescribed co-channel interference. In the hierarchical cell structure this is accomplished by assigning different sets of frequencies to different tiers. Within the tiers, frequency reuse is applied in order to maximize the usage of the radio spectrum.
Clearly, the deployment of the reuse concept requires planning efforts at the side of the network operator. The network operator has to plan the frequencies geographically in such a way that the aforedescribed mutual interference is minimized. For the high-tier levels (macro-cells) this is manageable, but when going down in the hierarchical levels, planning becomes more and more a difficult problem due to the unpredictable propagation conditions. In addition, when it comes to private radio systems like described in the aforementioned U.S. Pat. Nos. 5,428,668 and 5,526,402, which share the frequencies with the overlapping cellular environment, the operator has no direct control at all in assigning frequencies at the lowest tier.
Accordingly, proposals have been made for autonomous channel allocation methods in which the radio systems at the lowest tiers (public or private pico systems) autonomously determine which frequencies in the cellular radio spectrum can be reused in the local area for low-tier users. Typically, these low-tier systems periodically perform measurements on different carriers to xe2x80x9clearnxe2x80x9d the frequency usage of the overlapping (planned) high-tier system. With this information, selection can then be made of frequencies which are locally not used by the high-tier system, and which can therefore be reused by the low-tier system, thereby minimizing co-channel interference between the high-tier and the low-tier systems. Such an autonomous frequency allocation method is described in Applicant""s aforementioned co-pending patent applications entitled, xe2x80x9cMethod and System for Autonomously Allocating a Cellular Communications Between a Cellular Terminal and a Telephone Base Station,xe2x80x9d U.S. patent application Ser. No. 08/704,846, Aug. 28, 1996, and xe2x80x9cMethod and system for Autonomously Allocating a Frequency Hopping Traffic Channel in a Private Radio System, U.S. patent application Ser. No. 08/847,524, filed Apr. 24, 1997, which are incorporated herein by reference.
It should be understood that the reliability of the interference measurements performed using the techniques set forth in the above-identified patent applications depends upon the usage of the channels on which the measurements are made. If the channel carries broadcast information, a constant signal is on the channel and reliable interference measurements can be made. In a traffic channel context, however, the signal can be much more bursty, especially in FDMA/TDMA systems. Worst case scenarios in such systems are where the traffic channel is frequency hopping. As is understood to those skilled in the art, when the traffic load is low, the bursty signals on the frequencies have a low duty cycle and measurements become less and less reliable.
It is, therefore, an object of the present invention to provide a system and method to autonomously derive a set of frequencies for use in a low-tier radio system, which minimally interferes with an overlapping high-tier radio system that is applying frequency hopping. The system and method are preferably adaptive in the sense that if the high-tier system is replanned, the low-tier system will automatically adapt to the new conditions and select frequencies that will not compromise the operation of the high-tier system.
The present invention advantageously provides a system and a method to automatically select a frequency set in a low-tier radio communication system, sharing frequencies with an overlapping high-tier cellular radio system, that minimally interferes with the high-tier system employing frequency hopping. The present invention makes use of the broadcast information that is transmitted by the high-tier radio base stations on their broadcast control channels (BCCH), which contains not only the BCCH carrier frequencies of surrounding cells, but also information regarding the frequencies applied in the considered cell for frequency hopping traffic channels. With this broadcast information, which can be received on the downlink by high-tier and low-tier users alike, the low-tier system can derive the frequency planning of the high-tier system. In addition to this planning information, the low-tier system can measure the signal strengths on the BCCHs of surrounding cells. Together with the planning information, the low-tier system can then form a picture of the geographical usage of the frequencies, and can subsequently derive a frequency set for low-tier usage that minimally interferes with the overlapping high-tier system. The advantage with this technique is that only measurements on BCCH carriers have to be performed, which are non-hopping, have a constant transmit power, and have a continuous (non-bursty) signal.
A more complete appreciation of the present invention and the scope thereof can be obtained from the accompanying drawings which are briefly summarized below, the following detailed description of the presently-preferred embodiments of the invention, and the appended claims.